Torque distribution apparatus, torque distribution method, torque distribution value generation method, and program

ABSTRACT

A torque distribution apparatus acquires an instructed torque input and a motor efficiency map for motors; detects vehicular speed and drive wheel rotational speed; calculates based on the detected speeds, a relational expression of drive wheel slip rate and a friction coefficient; creates based on the relational expression, a performance curve expression indicating relations between torque and the drive wheel rotational speed, superimposes the performance curve expression on the motor efficiency map, creates an efficiency variation expression indicating for each vehicular speed, the torque and efficiency values of the motor efficiency map, and calculates a torque that optimizes efficiency from the efficiency variation expression; calculates based on the instructed torque and the torque optimizing efficiency, a torque distribution value for each motor; and controls torque distribution to each motor, within a range of the slip rate being 0 to 0.2 and based on the torque distribution values.

TECHNICAL FIELD

The present invention relates to a torque distribution apparatus, atorque distribution method, a torque distribution value generationmethod, and a program that distribute torque when multiple drive wheelsof a vehicle are driven. Nonetheless, use of the present invention isnot limited to the torque distribution apparatus, the torquedistribution method, the torque distribution value generation method,and the program.

BACKGROUND ART

Conventionally, electric vehicles (EV), which are mobile objects, areequipped with multiple motors, and as a means of distributing torquethat drives the drive wheels (wheels), the following technologies havebeen disclosed.

A first technology distributes torque by calculating power consumptionwith respect to combinations of motor torque and by obtaining a graphthat plots driving force distribution along the horizontal axis. In thisconfiguration, magnitude relations of the power Pout [kW] that can beoutput and the smallest value of power consumption (hereinafter, lowestpower consumption) when a motor torque combination realizing atransient, required driving force that is within a torque restriction isrealized are compared. If the power Pout [kW] that can be output isjudged to be greater than or equal to the lowest power consumption, themotor torque of the front-rear wheels that minimizes the powerconsumption is regarded to be the instructed torque as is (see, forexample, Patent Document 1).

A second technology distributes total torque to multiple motors suchthat torque is distributed equally to the 2 front drive wheels andtorque is distributed equally to the 2 rear drive wheels, by generatingand using a system efficiency map indicating the torque distributionratio that maximizes system efficiency (see, for example, PatentDocument 2).

A third technology retrieves based on the required driving power andvehicle speed, a map indicating relations among fuel consumption,discharged and charged power of the electrical storage device, andfront-rear wheel driving force distribution. From the extracted map, thedriving force distribution that minimizes fuel consumption with respectto the discharged and charged power of the electrical storage device isextracted, whereby driving force distribution maps before and afterimproved fuel efficiency are obtained (see, for example, Patent Document3).

In a fourth technology, based on vehicle speed and the required motordriving torque that corresponds to the required motor driving force andfurther based on the torque of each motor generator and efficiencycharacteristics corresponding to vehicle speed, a driving forcedistribution determining unit determines the distribution of drivingtorque among the motor generators. The driving torque distribution in alow output area and the driving torque distribution in a high outputarea are controlled using different patterns, and that which maximizesthe efficiency of the motor generators overall is adopted (see, forexample, Patent Document 4).

A fifth technology determines based on the total driving torque requiredof the left and the right front wheels and the rotational speed of themotor generators, the driving torque distribution between the left-frontwheel and the right-front wheel such that the driving efficiency of themotor generators overall is maximized. The fifth technology furtherdetermines the driving torque distribution between the right-front wheeland left-front wheel such that only one of the motor generators isdriven, according to the turning direction (see, for example, PatentDocument 5).

A sixth technology enables selection between wheel torque distributioncontrol that is based on energy efficiency (control of energyefficiency) and wheel torque distribution control that is based on thedistribution of load at each wheel (control of load distribution) (see,for example, Patent Document 6).

Such control that uses the required torque and energy efficiency asparameters to make the energy efficiency relatively high when drivingthe front and the rear wheels of 4-wheel drive vehicles by an electricmotor, is a known technique, as disclosed in, for example, PatentDocument 2. Further, distribution ratios of load at the front wheels andat the rear wheels, for example, are obtained from the height of thecenter of mass of the 4-wheel drive vehicle, the distance from thecenter of mass to the front wheel, the distance between the front wheelaxel and the rear wheel axel (wheel base), the width of the left andright tires (tread), the angular acceleration (horizontal acceleration)of the vehicle, acceleration of the vehicle in forward and backwarddirections, etc. and distribution ratios of load at the front wheels andat the rear wheels are caused to coincide with the distribution of loadbetween the front wheels and the rear wheel, whereby the distribution oftorque at the front wheels and the rear wheels is determined. Controlthat uses these parameters to obtain load distribution ratios for thefront wheels and the rear wheels and that determines torque distributionratios according to the load distribution ratios is a known technologyas exemplified by, for example, Patent Document 7.

-   Patent Document 1: Japanese Laid-Open Patent Publication No.    2006-180657-   Patent Document 2: Japanese Laid-Open Patent Publication No.    2006-345677-   Patent Document 3: Japanese Laid-Open Patent Publication No.    2007-37217-   Patent Document 4: Japanese Laid-Open Patent Publication No.    2007-313982-   Patent Document 5: Published Japanese-Translation of PCT    Application, Publication No. 2007/064025-   Patent Document 6: Japanese Laid-Open Patent Publication No.    2009-159682-   Patent Document 7: Japanese Laid-Open Patent Publication No.    2006-213130

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

Nonetheless, the technologies described in Patent Documents 1 to 7 areof a technological thought of distributing motor torque with an aim toincrease motor efficiency and do not make use a motor efficiency map anddistribute torque based on a torque value that is on the motorefficiency map and optimizes efficiency.

Further, with the technologies described in Patent Documents 1 to 3,each performs distribution with respect to the drive wheels in pairs,i.e., the 2 front wheels and the 2 rear wheels, and do not considerindependent control of each drive wheel. The technology described inPatent Document 4 is applied to a hybrid vehicle and is applicable toonly the in-wheel motors of the left and right rear wheel, which areindependent. The technology does not consider independent control ofeach drive wheel. With such technologies, for example, independentcontrol of 4 drive wheels is not possible and optical torquedistribution with respect to multiple drive wheels cannot be performed.

Moreover, none of the technologies described in Patent Documents 1 to 7give consideration to drive wheel (wheel) slippage and consequently,provide insufficient torque distribution that cannot achieve highefficiency. The state of slippage of drive wheels with respect to theroad surface varies consequent to factors such as vehicular speed andmore specifically to the rotational speed of the drive wheels, etc.Thus, without taking the state of drive wheel slippage intoconsideration, the efficiency obtained by distributing torque to each ofthe drive wheels cannot be improved and optimal torque distributioncannot be performed when the drive wheels are actually driven.Consequently, the efficiency of the drive system overall cannot beoptimized to the fullest extent.

Means for Solving Problem

To solve the problems above and achieve an object, a torque distributionapparatus according to the present invention distributes an inputinstructed torque to motors connected to drive wheels, and includes aninstructed torque acquiring unit that acquires the instructed torqueinput; an efficiency map acquiring unit that acquires a motor efficiencymap that corresponds to the motors; a vehicular speed detecting unitthat detects vehicular speed of a vehicle equipped with the motors; adrive wheel rotational speed detecting unit that detects drive wheelrotational speed of the drive wheels; a slip rate calculating unit thatbased on the vehicular speed and the drive wheel rotational speed,calculates slip rate at the drive wheels; a calculating unit that basedon the slip rate, creates an efficiency variation expression thatindicates efficiency values on a performance curve that indicatesrelations between the drive wheel rotational speed and torque, andcalculates a torque that optimizes efficiency from the efficiencyvariation expression on the performance curve; a distributing unit thatbased on the instructed torque and the torque optimizing efficiency,calculates a torque distribution value for each of the motors; and acontrol unit that based on the calculated torque distribution values,controls torque distribution to each of the motors.

Further according to the invention, a torque distribution method ofdistributing by a torque distribution apparatus, an input instructedtorque to motors connected to drive wheels, includes acquiring theinstructed torque input; acquiring a motor efficiency map thatcorresponds to the motors; detecting vehicular speed of a vehicleequipped with the motors; detecting drive wheel rotational speed of thedrive wheels; calculating based on the vehicular speed and the drivewheel rotational speed, slip rate at the drive wheels; creating based onthe slip rate, an efficiency variation expression that indicatesefficiency values on a performance curve that indicates relationsbetween the drive wheel rotational speed and torque, and calculating atorque that optimizes efficiency from the efficiency variationexpression on the performance curve; calculating based on the instructedtorque and the torque optimizing efficiency, a torque distribution valuefor each of the motors; and controlling based on the calculated torquedistribution values, torque distribution to each of the motors.

Further according to the invention, a torque distribution valuegeneration method of generating for each motor connected to a drivewheel and by a torque distribution value generating apparatus, a torquedistribution value for distributing an input instructed torque, includesacquiring the instructed torque input; acquiring a motor efficiency mapthat corresponds to the motors; detecting vehicular speed of a vehicleequipped with the motors; detecting drive wheel rotational speed of thedrive wheels; calculating based on the vehicular speed and the drivewheel rotational speed, slip rate at the drive wheels; creating based onthe slip rate, an efficiency variation expression that indicatesefficiency values on a performance curve that indicates relationsbetween the drive wheel rotational speed and torque, and calculating atorque that optimizes efficiency from the efficiency variationexpression on the performance curve; and obtaining based on theinstructed torque and the torque that optimizes efficiency, each torquedistribution value corresponding to the instructed torque and wheelspeed.

Further according to the invention, a program causes a computer toexecute any one among the disclosed methods.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a functional configuration of a torquedistribution apparatus according to an embodiment;

FIG. 2 is a flowchart of a procedure of a torque distribution processperformed by the torque distribution apparatus;

FIG. 3 is a diagram depicting a configuration of a vehicle;

FIG. 4 is a block diagram of a hardware configuration of the torquedistribution apparatus;

FIG. 5 is a diagram of an example of a motor efficiency map;

FIG. 6 is a diagram depicting relations between slip rate and frictioncoefficients;

FIG. 7 is a diagram depicting relations between rotational speed andtorque, taking the slip rate into consideration;

FIG. 8 is a diagram depicting a change curve depicted in FIG. 7superimposed on a motor efficiency map;

FIG. 9A is a diagram depicting relations between torque and efficiencythat change according to vehicular speed (part 1);

FIG. 9B is a diagram depicting relations between torque and efficiencythat change according to vehicular speed (part 2);

FIG. 10 is a diagram of relations between torque and efficiency;

FIG. 11A is a diagram depicting torque-efficiency characteristicsspecific to a motor (part 1);

FIG. 11B is a diagram depicting torque-efficiency characteristicsspecific to a motor (part 2);

FIG. 11C is a diagram depicting torque-efficiency characteristicsspecific to a motor (part 3);

FIG. 12A is a diagram depicting characteristics of a wheel, amongtorque-efficiency characteristics of an inversed U-type (part 1);

FIG. 12B is a diagram depicting characteristics of a wheel, amongtorque-efficiency characteristics of the inversed U-type (part 2);

FIG. 12C is a diagram depicting characteristics of a wheel, amongtorque-efficiency characteristics of the inversed U-type (part 3);

FIG. 12D is a diagram depicting characteristics of a wheel, amongtorque-efficiency characteristics of the inversed U-type (part 4);

FIG. 12E is a diagram depicting characteristics of a wheel, amongtorque-efficiency characteristics of the inversed U-type (part 5);

FIG. 12F is a diagram depicting characteristics of a wheel, amongtorque-efficiency characteristics of the inversed U-type (part 6);

FIG. 13A is a diagram depicting characteristics of a wheel, amongtorque-efficiency characteristics of a Δ-type (part 1);

FIG. 13B is a diagram depicting characteristics of a wheel, among thetorque-efficiency characteristics of the Δ-type (part 2);

FIG. 13C is a diagram depicting characteristics of a wheel, among thetorque-efficiency characteristics of the Δ-type (part 3);

FIG. 13D is a diagram depicting characteristics of a wheel, among thetorque-efficiency characteristics of the Δ-type (part 4);

FIG. 13E is a diagram depicting characteristics of a wheel, among thetorque-efficiency characteristics of the Δ-type (part 5);

FIG. 13F is a diagram depicting characteristics of a wheel, among thetorque-efficiency characteristics of the Δ-type (part 6);

FIG. 14A is a diagram of characteristics of a wheel, amongtorque-efficiency characteristics of a peak-type (part 1);

FIG. 14B is a diagram of the characteristics of a wheel, among thetorque-efficiency characteristics of the peak-type (part 2);

FIG. 14C is a diagram of the characteristics of a wheel, among thetorque-efficiency characteristics of the peak-type (part 3);

FIG. 14D is a diagram of the characteristics of a wheel, among thetorque-efficiency characteristics of the peak-type (part 4);

FIG. 14E is a diagram of the characteristics of a wheel, among thetorque-efficiency characteristics of the peak-type (part 5);

FIG. 14F is a diagram of the characteristics of a wheel, among thetorque-efficiency characteristics of the peak-type (part 6);

FIG. 15A is a diagram for describing torque distribution in a case of 4drive wheels (part 1);

FIG. 15B is a diagram for describing torque distribution in the case of4 drive wheels (part 2);

FIG. 15C is a diagram for describing torque distribution in the case of4 drive wheels (part 3);

FIG. 15D is a diagram for describing torque distribution in the case of4 drive wheels (part 4);

FIG. 15E is a diagram for describing torque distribution in the case of4 drive wheels (part 5);

FIG. 16A is a diagram for explaining variation differences amongtorque-efficiency characteristics;

FIG. 16B is a diagram for explaining deviation from the torque thatoptimizes efficiency in torque-efficiency characteristics;

FIG. 17 is a diagram for explaining dynamic torque distributionaccording to travel pattern;

FIG. 18 is a graph depicting optimal drive wheel count data set based onthe rotational speed-instructed torque;

FIG. 19 is a diagram for explaining overall efficiency;

FIG. 20 is a diagram depicting a torque-propulsion efficiency relationwhen the normal force is constant;

FIG. 21 is a diagram depicting states in which the normal force differsaccording to drive wheel;

FIG. 22 is a graph depicting torque-propulsion efficiency according tonormal force;

FIG. 23 is a diagram for explaining overall efficiency of the entirevehicle;

FIG. 24 is a block diagram depicting a functional configuration of thetorque distribution apparatus according to the second example;

FIG. 25A is a diagram depicting a calculation example of overallefficiency (part 1); and

FIG. 25B is a diagram depicting a calculation example of overallefficiency (part 2).

BEST MODE(S) FOR CARRYING OUT THE INVENTION

Preferred embodiments of a torque distribution apparatus, a torquedistribution method, a torque distribution value generation method, anda program according to the present invention will be described in detailwith reference to the accompanying drawings. Hereinafter, descriptionwill be given using “rotational speed” as the “rotational speed of adrive wheel”.

Embodiment

(Configuration of Torque Distribution Apparatus)

FIG. 1 is a block diagram of a functional configuration of the torquedistribution apparatus according to the embodiment. A torquedistribution apparatus 100 according to the embodiment uses a motorefficiency map and on the motor efficiency map, the torque optimizingefficiency is used to control torque distribution among multiple drivewheels.

The torque distribution apparatus 100 includes an instructed torqueacquiring unit 101, a vehicular speed detecting unit 102 a, a drivewheel speed detecting unit 102 b, a slip rate calculating unit 103, amotor efficiency map 104, an efficiency map acquiring unit 105, acalculating unit 106, a distributing unit 107, and a control unit 108.

The instructed torque acquiring unit 101 acquires an instructed torquefor driving a vehicle. In other words, the instructed torque acquiringunit 101 acquires the instructed torque input for driving n motors M(M1, M2, . . . Mn) respectively disposed in each drive wheel. In thepresent embodiment, description is given assuming that the same type ofmotor is used for each of the motors M.

The vehicular speed detecting unit 102 a detects the speed of thevehicle. The drive wheel speed detecting unit 102 b detects the speed ofthe drive wheels equipped on the vehicle. The drive wheel speed v_(w) iscalculated by the tire radius r×the drive wheel rotational speed ω(v_(w)=r×ω).

The slip rate calculating unit 103, as described hereinafter, calculatesthe slip rate at each drive wheel, based on the vehicle speed detectedby the vehicular speed detecting unit 102 a and the drive wheel speed(the drive wheel rotational speed and the tire diameter) detected by thedrive wheel speed detecting unit 102 b. The motor efficiency map 104, asdescribed hereinafter with reference to FIG. 5, is a map depictingrelations between speed and torque for each motor M; and on this map,based on torque and speed, a substantially linear performance curve canbe drawn. The efficiency map acquiring unit 105 acquires the motorefficiency map 104 that corresponds to the motor M.

As a means of acquiring the motor efficiency maps, 1. motor efficiencymaps prepared in advance by the manufacturer of the motor or of thevehicle are retained in memory; 2. motor efficiency maps are createdduring travel of the vehicle, etc. may be considered.

The calculating unit 106 creates based on the slip rate calculated bythe slip rate calculating unit 103, an efficiency variation expressionthat indicates an efficiency value on a performance curve indicatingrelations between rotational speed and torque; and calculates a torquethat optimizes efficiency in the efficiency variation expression on theperformance curve.

The distributing unit 107, as described hereinafter, calculates torquedistribution value for each motor M, based on the instructed torqueacquired by the instructed torque acquiring unit 101 and the torqueoptimizing efficiency calculated by the calculating unit 106. Thecontrol unit 108 controls the torque distributed to each of the motors Mbased on the torque distribution values calculated by the distributingunit 107.

(Torque Distribution Process)

FIG. 2 is a flowchart of a procedure of a torque distribution processperformed by the torque distribution apparatus. The instructed torqueacquiring unit 101 acquires an instructed torque T that is input fromthe accelerator pedal and are for driving each of the motors M (M1, M2,. . . Mn) disposed in the drive wheels (step S201). The vehicular speeddetecting unit 102 a detects the vehicular speed of the vehicle (stepS202), and the drive wheel speed detecting unit 102 b detects the wheelspeed at the drive wheels (step S203). The slip rate calculating unit103 calculates the slip rate at each drive wheel, using the vehicularspeed and the drive wheel speed (drive wheel rotational speed and tirediameter) (step S204).

The efficiency map acquiring unit 105 acquires the motor efficiency map104 corresponding to the motor M (step S205). The calculating unit 106creates based on the wheel speed detected by the drive wheel speeddetecting unit 102 b and the slip rate calculated by the slip ratecalculating unit 103, an efficiency variation expression that indicatesan efficiency value on a performance curve indicating relations betweenthe rotational speed and torque; and calculates a torque that optimizesefficiency To in the efficiency variation expression on the performancecurve (step S206). The distributing unit 107 calculates a torquedistribution value for each motor M, based on the instructed torque Tacquired by the instructed torque acquiring unit 101 and the torque thatoptimizes efficiency To calculated by the calculating unit 106 (stepS207). The control unit 108 controls the distribution of torque to eachof the motors M, based on the torque distribution values calculated bythe distributing unit 107 (step S208).

Although in general, the performance curve on the motor efficiency mapis linear, in actuality, if torque distribution changes, drive wheeltorque varies and according to the drive wheel torque variation, therotational speed of the drive wheel varies. Therefore, under thecondition that the vehicular speed remains constant, the performancecurve on the motor efficiency map is not a straight line, but rather aslope (strictly speaking, a curve) as described hereinafter withreference to FIG. 5. Efficiency values for the torque values on theperformance curve can be represented as torque-efficiencycharacteristics. Since the torque-efficiency characteristics are curvesas described hereinafter with reference to FIG. 10, the torque by whichthe efficiency is maximized appears. This torque is called the torquethat optimizes efficiency To. Taking the torque that optimizesefficiency To as a standard, all of the instructed torque T isdistributed to the n motors M by a given torque distribution.

An example of the distribution of torque to the n motors M by thedistributing unit 107 will be described. The distributing unit 107distributes all of the torque that optimizes efficiency To to only aportion of the motors M among the n motors, or to all of the motors M,or equally distributes the instructed torque T such that the torquedistribution values of a portion of the motors M maximally approach thetorque that optimizes efficiency To.

Here, the efficiency variation expression indicating efficiency valueson the performance curve that indicates relations between wheel speedand slip rate as well as between rotational speed and torque is obtainedby the following procedure.

1. Detect current vehicular speed

2. Detect current drive wheel speed

3. Calculate slip rate

4. Detect current torque from motor driving current

5. Calculate performance curve expression (using expression (8)described hereinafter)

6. Draw performance curve on efficiency map, along performance curve,acquire multiple points of combinations of torque value and efficiencyvalue

7. Create efficiency variation expression from multiple points of torquevalue and efficiency value

Here, the greater the number of points, the greater the accuracy of theapproximation expression of the efficiency variation expression is.

A torque Td is μ·N·r (μ: friction coefficient of road surface and tier;N: normal force; r: tire radius) (expression (5) described hereinafter)and therefore, if the torque Td changes, the friction coefficient μvaries. If the friction coefficient μ varies, a slip rate λ varies(depicted in FIG. 6). If acceleration and deceleration are not great,variation of a rotational speed v is small and therefore, if the sliprate λ varies, the rotational speed ω varies. In other words, accordingto the torque value given to a drive wheel, the rotational speed of thedrive wheel varies. This relation is described hereinafter withreference to expression (8) and FIG. 7. To use expression (8), therelation between slip rate and friction coefficient (describedhereinafter with reference to expression (6)) is used. Concerning thisrelation, expression (6) for the traveled road surface is acquired froma server that is outside the vehicle, or is created by the vehicle.

Equation (6) for the relation between slip rate and friction coefficientis created by the vehicle according to the following procedure.

1. Detect current vehicular speed and drive wheel rotational speed, andobtain slip rate λ

2. Obtain torque value from current motor driving current, and calculateμ value from Td=μ·N·r

3. Obtain multiple points for λ and μ values during travel, create μ-λcharacteristics of FIG. 6, and generate expression (6).

The torque value is obtained by multiplying a preliminarily known torqueconstant by the driving current. Further, in this case, since travelnormally occurs without idle rotation of the tires, a λ value and a μvalue (in FIG. 6, λ is 0.2 or less) that are within a range that doesnot exceed a peak portion of μ can be detected. Although a point in arange that does not exceed the peak portion of μ (in FIG. 6, λ is 0.2 orgreater) cannot be detected, in the present invention, a required areais a range that does not exceed the peak portion of μ and therefore, ifμ-λ characteristics of the area are created and expression (6) isgenerated, no problem arises.

The torque distribution apparatus 100 according to the embodimentdescribed above takes into consideration the calculated slip rate whencalculating the torque that optimizes efficiency To on the motorefficiency map 104. Although the state of drive wheel slippage withrespect to the road surface varies consequent to factors such asvehicular speed and more specifically to the rotational speed of drivewheels, etc., by taking the state of slippage of the drive wheels intoconsideration, the efficiency obtained by distributing torque to each ofthe drive wheels can be improved; optimal torque distribution can beperformed when the drive wheels are actually driven; and since themotors can be driven in an area where the motor efficiency is high, theefficiency of the drive system overall can be optimized to the fullestextent. This drive system points to a configuration related to vehicledriving by a motor and inverter.

EXAMPLE First Example

A first example of according to the present invention will be described.In the first example, an example of application of the torquedistribution apparatus in a vehicle equipped with in-wheel motors thatare built into 4 drive wheels respectively and independently drive thedrive wheels. In this case, the number of motors M used is 4, M1 to M4.A 3-phase alternating current motor or a DC motor can be used for themotors M. In the example hereinafter, the same motor is used in each ofthe 4 drive wheels. As described hereinafter, the drive wheels are notlimited to 4 and the present invention is applicable to 2, 3, 5 or moredrive wheels

(Configuration of Vehicle)

FIG. 3 is a diagram depicting a configuration of the vehicle. A vehicle300 is a 4-wheel drive vehicle having left and right front drive wheelsFL, FR and left and right rear drive wheels RL, RR. These 4 drive wheelsFL, FR, RL, RR are equipped with the in-wheel motors M1 to M4,respectively and are independently driven.

The motors M1 to M4 are each equipped with an inverter INV for drivingthe motors. The inverters INV drive the motors M1 to M4, under thecontrol of a controller (ECU) 301. The controller 301 receives input ofvarious types of information and consequent to the distribution oftorque, drives the motors M1 to M4.

Input to the controller 301 includes the following. The steering angleis input from a steering wheel 302. The instructed torque is input froman accelerator pedal 303. The braking amount is input from a brake pedal304. The parking brake amount is input from a parking brake 305. Thegear position such as reverse, neutral and drive is input by gears 306.

Further, the drive wheels FL, FR, RL, RR are each equipped with a sensor307 a to 307 d that detects a rotational speed V. The rotational speedsVfl, Vfr, Vrl, Vrr of the drive wheels FL, FR, RL, RR are input to thecontroller 301. The drive wheels FL, FR, RL, RR are each equipped with asensor 308 a to 308 d that detects the normal force N subjected to thetires from the ground. The normal force Nfl, Nfr, Nrl, Nrr of each ofthe drive wheels FL, FR, RL, RR is input to the controller 301.

The vehicle 300 is equipped with an acceleration sensor 309 and thedetected acceleration is input to the controller 301. The vehicle 300 isfurther equipped with a yaw rate sensor 310 and the detected yaw rate isinput to the controller 301.

The controller 301 drives the drive wheels FL, FR, RL, RR, based on theabove input. A control signal for driving, is supplied to the motors M1to M4, via the inverter INV, and suitably distributes torque to each ofthe drive wheels FL, FR, RL, RR.

A battery 312 supplies power to the entire vehicle 300. In particular,the battery 312 is a drive source for driving the motors M1 to M4 of thedrive wheels FL, FR, RL, RR, via the inverter INV. A secondary cell suchas a nickel metal hydride and a lithium ion secondary cell, or a fuelcell can be adopted as the battery 312.

During regeneration at the vehicle 300, the inverter INV converts thealternating voltage generated by the motors M1 to M4 into direct voltageand can supply the resulting direct voltage to the battery 312.Regeneration is the generation of electric power when the driver of thevehicle 300 manipulates the brake pedal 304, and the generation ofelectric power by an easing of the force applied to the acceleratorpedal 303 during travel.

Driving efficiency is expressed by driving efficiency η=motor Moutput/power supplied by battery 312=(T×ω)/(V×I).

(Hardware Configuration of Torque Distribution Apparatus)

A hardware configuration of a torque distribution apparatus 400 will bedescribed. FIG. 4 is a block diagram of a hardware configuration of thetorque distribution apparatus. In FIG. 4, the torque distributionapparatus 400 includes a CPU 401, ROM 402, RAM 403, a communication I/F415, a GPS unit 416, and various sensors 417, respectively connected bya bus 420.

The CPU 401 governs overall control of the torque distribution apparatus400. The ROM 402 stores programs such as a boot program and the torquedistribution program and can further store the motor efficiency maps.The RAM 403 is used as a work area of the CPU 401. In other words, theCPU 401 uses the RAM 403 as a work area and executes programs stored onthe ROM 402 to thereby govern overall control of the torque distributionapparatus 400.

The communication I/F 415 is wirelessly connected to a network andfunctions as an interface of the torque distribution apparatus 400 andthe CPU 401. Among communication networks functioning as the network arepublic line and mobile telephone networks, as well as dedicated shortrange communication (DSRC), LANs, and WANs. The communication I/F 415is, for example, a module for connecting to public lines, an ETC unit,an FM tuner, a vehicle information and communication system(VICS)/beacon receiver and the like.

The GPS unit 416 receives signals from GPS satellites, and outputsinformation indicating the current position of the vehicle. Informationoutput by the GPS unit 416 is used in conjunction with values output bythe various sensors 417 described hereinafter when the current positionof the vehicle is calculated by the CPU 401. Information indicating thecurrent position is, for example, information that identifies 1 point onmap data such as longitude/latitude, altitude, and the like.

Here, in a case where the slip rate and friction coefficient (μ-λ)characteristic of the traveled road surface is acquired from a serveroutside the vehicle, the communication I/F 415 and the GPS unit 416 areused. The various sensors 417 are used in the detection of vehicularspeed and the normal force. The vehicular speed, for example, isdetected by the following methods.

1. Integration of acceleration sensor output

2. Calculated from rotational speed of non-driving wheels

3. Calculated from traveled distance per unit time obtained by GPSand/or other positioning sensors

To detect the normal force, load sensors disposed at each tire are usedor the following methods are used.

1. Obtain displacement of barycentric position from output ofaccelerator sensor and calculate load balance of front wheels and rearwheels

2. Obtain displacement of barycentric position from output of yaw ratesensor and calculate load balance of front wheels and rear wheels

3. Obtain displacement of barycentric position from output ofinclination sensor (gyro) and calculate load balance of front wheels andrear wheels, as well as right wheels and left wheels

Functions of the calculating unit 106, the distributing unit 107, andthe control unit 108 of the torque distribution apparatus 100 depictedin FIG. 1 are implemented by controlling each of the components in thetorque distribution apparatus 400 by executing on the CPU 401, a givenprogram by using the programs and data stored in the ROM 402 and the RAM403 of the torque distribution apparatus 400.

(Torque Distribution Control by Torque Distribution Apparatus)

The torque distribution apparatus 400 of the present example performsoptimization such that the efficiency of the drive system is maximized.The torque provided to the drive wheels is indicated as T1, T2, T3, T4respectively for the drive wheels; the efficiency is indicated as η1,η2, η3, η4; and the overall efficiency η of the 4 wheels is express byexpression (1).η=(T1·η1+T2·η2+T3·η3+3+T4·η4)/T  (1)(total driving torque T=T1+T2+T3+T4)

FIG. 5 is a diagram of an example of the motor efficiency map. Thehorizontal axis represents rotational speed and the vertical axisrepresents torque. The following may be considered when in FIG. 5,efficiency that is obtained from a performance curve C that is linearwhen the vehicle is traveling at a constant speed, is used for drivewheel selection.

(1) torque driving by 4 wheels

(2) torque driving by 2 wheels

(3) torque driving by only 1 wheel

(1) In the case of torque driving by 4 wheels (distribute ¼ (0.25) oftorque to each of the 4 wheels)η1=0.25·0.77+0.25·0.77+0.25·0.77+0.25·0.77=0.77

(2) In the case of torque driving by 2 wheels (distribute ½ (0.5) oftorque to each of the 2 wheels)η2=0.5·0.83+0.5·0.83+0+0=0.83

(3) In the case of torque driving by only 1 wheel (distribute all (1) oftorque to only 1 wheel)η3=1·0.72+0+0+0=0.72According to the description above, it can be seen that torqueefficiency improves by performing torque distribution that has a lot oftorque in an area where efficiency is high.

Here, relations between torque and rotational speed will be described.

A motion expression of the drive wheels and the driving force of a drivewheel are represented as expressions (2), (3), (4).

$\begin{matrix}{{J_{w}\frac{\mathbb{d}\omega}{\mathbb{d}t}} = {T_{m} - T_{d}}} & (2) \\{F_{d} = {\mu \times N}} & (3) \\{T_{d} = {F_{d} \times r}} & (4)\end{matrix}$

(Tm: instructed torque for motor; Td: driving torque of drive wheel; Fd:driving force; Jw: moment of inertia of drive wheel; μ: frictioncoefficient of road surface, tire; N: normal force; r: tire radius)

Here, the driving torque of the drive wheel means the torque of themotor equipped in the drive wheel.

If abrupt acceleration or deceleration are not performed, variations inspeed are gradual and therefore, variation of the rotational speed islow and expression (5) below is obtained.dω/dt≈0(5)That is, an instructed torque Tm for the motor and the driving torque Tdof the drive wheel are approximately equal,∴Tm≈Td=Fd·r=μ·N·rHereinafter, description will be continued assuming that variation ofthe vehicular speed is gradual, and the instructed torque Tm of themotor and the driving torque Td of the drive wheel are approximatelyequal.

FIG. 6 is a diagram depicting relations between slip rate and frictioncoefficients. The horizontal axis represents the slip rate λ. Thevertical axis represents the friction coefficient μ. The slip rate λ andthe friction coefficient μ have the relation depicted in FIG. 6 and canbe approximated by expression (6) below. In the graph depicted in FIG.6, the friction coefficient μ is greatest when the slip rate λ is 0.2.When the slip rate λ is 1, this corresponds to idle rotation of thedrive wheel. By performing control to suppress the slip rate λ to bewithin a range of 0 to 0.2, travel can be performed without idlerotation of the drive wheel. Further, concerning the characteristicsdepicted in FIG. 6, the maximum value of μ or the value of λ, which isthe maximum of μ, vary consequent to the state of the tire and/or roadsurface. Even in this case, by changing the values of the parameters B,C, D, E in expression (6) approximation can be performed. However, ingeneral, since sudden changes in the surface of the tires and asphaltroads are rare, changes in the μ-λcharacteristics during travel aregradual.

$\begin{matrix}{{\mu = {D \times {\sin\left( {{C \times {\tan^{- 1}\left( {B \times \left( {1 - E} \right) \times \lambda} \right)}} + {\frac{E}{B} \times {\tan^{- 1}\left( {B \times \lambda} \right)}}} \right)}}}\left( {{B = 10},{C = 1.5},{D = 0.8},{E = 0.2}} \right)} & (6)\end{matrix}$Further, λ=(r·ω−v)/(r·ω)=1−v/(r·ω)  (7)

Therefore, Td is expressed by expression (8).Td=Fd·r=μ·N·r=D·sin(C·tan⁻¹(B·(1·E)·λ)+(E/B)·tan⁻¹(B·λ))·N·r=D·sin(C·tan⁻¹(B·(1·E)·(1·(v/(r·ω)))+(E/B)·tan⁻¹(B+(1−(v/(r·ω))))·N·r  (8)

If abrupt acceleration or deceleration is not performed, variations inthe speed are gradual and therefore, a vehicular speed v lookssubstantially constant and the relation between Td and ω can be obtainedby expression (8).

FIG. 7 is a diagram depicting relations between the rotational speed andtorque, taking the slip rate into consideration. The torque androtational speed calculated based on the expression above are depicted.Here, the normal force N: 400 [kg]×9.8 [m/s²]; tire radius r: 0.3 [m];the vehicular speed v=25, 50, 75, 100 [km/h] are assumed.

Therefore, if the torque of the drive wheel varies consequent to achanging of the torque distribution, the rotational speedcorrespondingly varies. The vehicular speed curves respectively depictedin FIG. 7 are not straight lines but rather gradually tilt as torqueincreases and are change curves that become saturated at the maximumtorque.

FIG. 8 is a diagram depicting the change curve depicted in FIG. 7superimposed on a motor efficiency map. The horizontal axis representsthe rotational speed ω and the vertical axis represents the torque Td.In the present example, the motor efficiency map includes not only thecharacteristics of the motor M, but also depicts characteristics thatinclude the characteristics (efficiency) of the inverter INV included inthe drive system.

As depicted in FIG. 8, in the performance curve C of a given speed(e.g., 75 [km/h]), when the torque of 1 drive wheel is at a point_a andthe torque of the drive wheel varies greatly consequent to changing thetorque distribution, other points such as point_b and point_c on theperformance curve C move. In this case, since the performance curve C issloped, the value of the rotational speed ω also increases. Therefore,if torque distribution is performed without taking variation of therotational speed ω into consideration, proper operating points are notknown and consequently, margins of error arise in the efficiency values.Therefore, as depicted in FIG. 8, by drawing performance curves for eachvehicular speed on motor efficiency map, and obtaining torque-efficiencyrelations, even when the torque distribution varies, the efficiency canbe properly calculated.

FIGS. 9A and 9B are diagrams depicting relations between torque andefficiency that change according to vehicular speed. FIG. 9A depictstorque-efficiency characteristics on a performance curve when thevehicular speed is 50 [km/h]. FIG. 9B depicts torque-efficiencycharacteristics on a performance curve when the vehicular speed is 75[km/h].

Further, the efficiency η obtained by a sixth order approximation of theperformance curve corresponding to FIG. 9A is:η=−1.7088E−14Td ⁶+1.8521E−11Td ⁵−7.9786E−09Td ⁴+1.7336E−06Td³−2.0447E−04Td ²+1.1782E−02Td+4.4673E−01  (9)Further, the efficiency η obtained by a sixth order approximation of theperformance curve corresponding to FIG. 9B is:η=1.1253E−14Td ⁶−1.0197E−11Td ⁵+3.2448E−09Td ⁴−3.5952E−07Td³−2.6286E−05Td ²+7.8911E−03Td+4.9954E−01  (10)

By substituting a value for the torque Td in the approximationexpression above, the efficiency η can be obtained. According toexpression (1), in the case of 4-wheel drive, the greatestT1·η1+T2·η2+T3·η3+T4·η4 within a conditional range of T1+T2+T3+T4=T(instructed torque), is the optimal efficiency.

A calculation method of the slip rate will be described. The slip rate λis defined by expression (11) below.

$\begin{matrix}{\lambda = {\frac{v_{w} - v}{v_{w}} = \frac{{r \times \omega} - v}{r \times \omega}}} & (11)\end{matrix}$

Here, (v: vehicular speed; v_(w): drive wheel speed; ω: drive wheelrotational speed; r: tire radius); and since the greater among v andv_(w) is the denominator, during acceleration, the denominator is v_(w)as above and during deceleration, the denominator is v. Supplementaldescription will be given concerning differences between the vehicularspeed, the drive wheel speed, and the drive wheel rotational speed. Bymultiplying the rotational speed of the tire by the tire radius,traveling speed of the tire is obtained. When the motors are driven andthe vehicle is traveling, the speed of the tires is slower than thespeed of the vehicle. Meanwhile, when braking of the motors occurs whilethe vehicle is traveling, the speed of the tires is slower than thespeed of the vehicle. The slip rate indicates the relation between tirespeed and vehicular speed and is expressed by expression (12).slip rate=(wheel speed−vehicular speed)/the greater among vehicularspeed and wheel speedλ=(v _(w) −v)/Max(v _(w) ,v)  (12)For the wheel of a motor that is neither driven nor braked, the sliprate is approximately zero and therefore, the speed of this wheel isapproximately equal to the vehicular speed (v_(w)≈v).

The rotational speed of the drive wheel can be calculated using a pulsedoutput signal of the resolver of the motor M, an encoder, a Hallelement, and the like. To obtain the vehicular speed, 1. since the sliprate of a non-driving wheels is approximately zero, the speed of anon-driving wheel can be detected as the vehicular speed; 2. the outputof the acceleration sensor is integrated and the vehicular speed isobtained; and 3. the vehicular position is detected by a sensor and thespeed at which a distance is traveled per unit time is obtained, can beconsidered.

(Example of Torque Distribution)

FIG. 10 is a diagram of relations between torque and efficiency. Similarto FIGS. 9A and 9B, the horizontal axis represents torque and thevertical axis represent efficiency. As depicted in FIG. 10, the point onthe performance curve where the efficiency η is greatest is regarded asthe torque that optimizes efficiency To. Further, on the performancecurve, the efficiency that corresponds to the torque that is twice thetorque that optimizes efficiency To is indicated as 2To.

(Characteristics of Performance Curve on Motor Efficiency Map)

Here, torque distribution for torque-efficiency characteristics specificto each motor will be described. As with the torque-efficiencycharacteristics above in FIGS. 9A and 9B, each has a curve specific toeach motor M. FIGS. 11A to 11C are diagrams depicting torque-efficiencycharacteristics specific to each motor. FIG. 11A is abbreviated as aninversed U-type; FIG. 11B is abbreviated as a Δ-type; and FIG. 11C isabbreviated as a peak-type. For simplification, torque distribution inthe case of 2 drive wheels is considered, where the drive wheels areequipped with motors having the same characteristics. Based onexpression (1), the efficiency η when all the instructed torque T isdistributed among 2 wheels is:η=(T1·η1+T2·η2)/T  (13)(torque of drive wheel 1: T1; corresponding efficiency: η1; torque ofdrive wheel 2: T2, corresponding efficiency: η2)

For example, if the instructed torque T is 160[Nm], (T1,T2)=(100,60),(80,80), and the like, although there are many combinations, bysubstituting the torque value of each into expression (13) above, theefficiency can be calculated. Thus, the combination of torque valuesthat maximizes efficiency merely has to be selected. An example will bedescribed below. FIGS. 12A to 14F depict the efficiency η1 of drivingwheel 1×the torque distribution ration T1/T, the efficiency η2 ofdriving wheel 2×the torque distribution ration T2/T, and characteristicsof the total efficiency η, when with respect to the motors having thetorque-efficiency characteristics depicted in FIGS. 11A to 11C, thetorque distribution to drive wheel 1 and drive wheel 2 is changed withrespect to all the instructed torque T.

FIGS. 12A to 12F are diagrams depicting characteristics for torquedistribution to two drive wheels equipped with motors having thetorque-efficiency characteristics of the inversed U-shape depicted inFIG. 11A. In FIG. 12A, the instructed torque T is 100[Nm] and theefficiency η is greatest when (T1,T2)=(0,100), (100,0). The totalefficiency is the characteristic obtained by substitution intoexpression (13). In FIG. 12B, the instructed torque T is 120[Nm], andthe efficiency η is greatest when (T1,T2)=(0,120), (120,0). In FIG. 12C,the instructed torque T is 140[Nm], and the efficiency η is greatestwhen (T1,T2)=(0,140), (140,0). In FIG. 12D, the instructed torque T is160[Nm], and the efficiency η is greatest when (T1,T2)=(80,80). In FIG.12E, the instructed torque T is 180[Nm], the efficiency η is greatestwhen (T1,T2)=(90,90). In FIG. 12F, the instructed torque T is 200[Nm],and the efficiency η is greatest when (T1,T2)=(100,100).

FIGS. 13A to 13F are diagrams depicting characteristics for torquedistribution to two drive wheels equipped with motors having thetorque-efficiency characteristics of the inversed Δ-shape depicted inFIG. 11B. In FIG. 13A, the instructed torque T is 100[Nm], and theefficiency η is greatest when (T1,T2)=(0,100), (100,0). In FIG. 13B, theinstructed torque T is 120[Nm], and the efficiency η is greatest when(T1,T2)=(0,120), (120,0). In FIG. 13C, the instructed torque T is140[Nm], and the efficiency η is greatest when (T1,T2)=(40,100),(100,40). In FIG. 13D, the instructed torque T is 160[Nm], and theefficiency η is greatest when (T1,T2)=(60,100), (100,60). In FIG. 13E,the instructed torque T is 180[Nm], and the efficiency η is greatestwhen (T1,T2)=(80,100), (100,80). In FIG. 13F, the instructed torque T is200[Nm], and the efficiency η is greatest when (T1,T2)=(100,100).

FIGS. 14A to 14F are diagrams depicting characteristics for torquedistribution to two drive wheels equipped with motors having thetorque-efficiency characteristics of the inversed peak-type depicted inFIG. 11C. In FIG. 14A, the instructed torque T is 100[Nm], and theefficiency η is greatest when (T1,T2)=(0,100), (100,0). In FIG. 14B, theinstructed torque T is 120[Nm], and the efficiency η is greatest when(T1,T2)=(20,100), (100,20). In FIG. 14C, the instructed torque T is140[Nm], and the efficiency η is greatest when (T1,T2)=(40,100),(100,40). In FIG. 14D, the instructed torque T is 160[Nm], and theefficiency η is greatest when (T1,T2)=(60,100), (100,60). In FIG. 14E,the instructed torque T is 180[Nm], and the efficiency η is greatestwhen (T1,T2)=(80,100), (100,80). In FIG. 14F, the instructed torque T is200[Nm], and the efficiency η is greatest when (T1,T2)=(100,100).

Thus, the combination that maximizes the efficiency η is any of thefollowing:(T1,T2)=(0,T),(T,0),(To,T−To),(T−To,To),(T/2,T/2)  (14)(To: torque optimizing efficiency)Thus, even if the shape of the curve of the torque-efficiencycharacteristics is any one of the types in FIGS. 11A to 11C, among thecombinations of expression (14), the presence of the combination thatmaximizes the efficiency η is focused on.

In other words, even when the shape of the curve of thetorque-efficiency characteristics is unclear, the combination thatmaximizes the result of calculating the combinations depicted inexpression (14) is the combination of torque distribution that maximizesthe efficiency η. When the shape of the curve of the torque-efficiencycharacteristics is complicated, although the combination maximizingefficiency may be a combination other than those among expression (14),for complicated characteristics excluding those having many points ofreverse curvature, the combination that maximizes efficiency among thecombinations indicated by expression (14) is present. In other words,although the combinations of torque distribution are countless, bymerely calculating the combinations indicated by expression (14), anoptimal torque distribution value can be obtained. In the example above,although description is given for torque distribution for 2 wheels,torque distribution for multiple wheels such as 4, etc. is the same.

Thus, in the distributions depicted in FIGS. 15A to 15E describedhereinafter, a combination that maximizes the efficiency η is present.Further, since the motor efficiency maps indicating thetorque-efficiency characteristics of the inversed U-type are numerous,simplification of the torque distribution described hereinafter withreference to FIGS. 16A and 16B is possible.

Assuming that the number of motors M is n (n=natural number), thedistributing unit 107:

(1) Distributes all of the instructed torque T to the torquedistribution value of one of the motors M, when the instructed torque Tis less than the torque that optimizes efficiency To.

(2) Performs torque distribution by any one among (a) to (c) below, whenthe instructed torque T is greater than or equal to the torque thatoptimizes efficiency To and is less than n times the torque thatoptimizes efficiency To. In this case, among (a) to (c), that which hasthe optimum drive system efficiency is selected.

(a) Distributes the torque that optimizes efficiency To to therespective torque distribution values of a portion of the motors M andfurther distributes the remainder obtained by dividing the instructedtorque T by the torque that optimizes efficiency To to one of themotors, or equally distributes the remainder to the n motors M.(b) Distributes the torque that optimizes efficiency To to therespective torque distribution values of a portion of the motors M andequally distributes to the respective torque distribution values ofother motors M, the torque that remain after distribution to the portionof the motors M.(c) Distributes the instructed torque T equally to each of the motors M.

(3) When the instructed torque T is greater than or equal to n times thetorque that optimizes efficiency To, distributes the torque thatoptimizes efficiency To to the respective torque distribution values ofthe n motors M, and further selects the combination having the optimumdrive system efficiency, among the one or the n motors M to which theremainder obtained by dividing the instructed torque T by the torquethat optimizes efficiency To, is divided and equally distributed.

FIGS. 15A to 15E are diagrams for describing torque distribution in thecase of 4 drive wheels. Distribution examples in which the distributingunit 107 distributes torque to each of the n motors M, where n=4, willbe described.

(When T<To)

As depicted in FIG. 15A, when the instructed torque T is less than thetorque that optimizes efficiency To, the instructed torque T isdistributed to the torque distribution value of one motor.

(When To≦T<2To)

As depicted in FIG. 15B, when the instructed torque T is greater than orequal to the torque that optimizes efficiency To and less than twice thetorque that optimizes efficiency To, from among (a) to (c), thecombination having the optimum drive system efficiency is selected.

(a) Distribute the instructed torque T to the torque distribution valueof one motor.

(b) Distribute the torque that optimizes efficiency To the torquedistribution value of one motor and distribute the remaining torque tothe torque distribution value of another (one) motor.

(c) Distribute ½ of the instructed torque T to the torque distributionvalues of two motors.

(When 2To≦T<3To)

As depicted in FIG. 15C, when the instructed torque T is greater than orequal to twice the torque that optimizes efficiency To and less than 3times the torque that optimizes efficiency To, from among (a) to (e),the combination having the optimum drive system efficiency is selected.

(a) Distribute the torque that optimizes efficiency To to the torquedistribution value of one motor and distribute the remaining torque tothe torque distribution value of another (one) motor.

(b) Distribute ½ of the instructed torque T to the torque distributionvalues of two motors.

(c) Distribute the torque that optimizes efficiency To to the torquedistribution values of two motors and distribute the remaining torque tothe torque distribution value of another (one) motor.

(d) Distribute the torque that optimizes efficiency To to the torquedistribution value of one motor and distribute ½ of the remaining torqueto the torque distribution values of two motors.

(e) Distribute ⅓ of the instructed torque T to the torque distributionvalues of three motors.

(When 3To≦T<4To)

As depicted in FIG. 15D, when the instructed torque T is greater than orequal to 3 times the torque that optimizes efficiency To and less than 4times the torque that optimizes efficiency To, from among (a) to (g),the combination having the optimum drive system efficiency is selected.

(a) Distribute the torque that optimizes efficiency To to the torquedistribution values of two motors and distribute the remaining torque tothe torque distribution value of one of the remaining motors.

(b) Distribute the torque that optimizes efficiency To to the torquedistribution value of one motor, and distribute ½ of the remainingtorque to the torque distribution values of two motors.

(c) Distribute ⅓ of the instructed torque T to the torque distributionvalues of three motors.

(d) Distribute the torque that optimizes efficiency To to the torquedistribution values of three motors and distribute the remaining torqueto the torque distribution value of the remaining motor.

(e) Distribute the torque that optimizes efficiency To to the torquedistribution values of two motors and distribute ½ of the remainingtorque to the torque distribution values of the remaining two motors.

(f) Distribute the torque that optimizes efficiency To to the torquedistribution value of one motor, and distribute ⅓ of the remainingtorque to the torque distribution values of the remaining three motors.

(g) Distribute ¼ of the instructed torque T to the torque distributionvalues of 4 motors.

(When 4To≦T)

As depicted in FIG. 15E, the instructed torque T is greater than orequal to 4 times the torque that optimizes efficiency To, from among (a)to (d), the combination having the optimum drive system efficiency isselected.

(a) Distribute the torque that optimizes efficiency To to the torquedistribution values of three motors and distribute the remaining torqueto the torque distribution value of the remaining one motor.

(b) Distribute the torque that optimizes efficiency To to the torquedistribution values of two motors, and distribute ½ of the remainingtorque to the torque distribution values of the remaining two motors.

(c) Distribute the torque that optimizes efficiency To to the torquedistribution value of one motor and distribute ⅓ of the remaining torqueto the torque distribution values of the remaining three motors.

(d) Distribute ¼ of the instructed torque T to the torque distributionvalues of four motors.

Expression of the distribution examples as expressions is as follows.

(When T<To)T1=T,T2=T3=T4=0(When To≦T<2To)

Efficiency is calculated by the following three methods (a) to (c), andthe combination that maximizes efficiency is selected.T1=To+(T−To),T2=T3=T4=0  (a)T1=To,T2=To−(2To−T),T3=T4=0  (b)T1=T2=To−(2To−T)/2,T3=T4=0  (c)(When 2To≦T<3To)

Efficiency is calculated by the following five methods (a) to (e), andthe combination that maximizes efficiency is selected.T1=To+(T−2To),T2=To,T3=T4=0  (a)T1=T2=To+(T−2To)/2,T3=T4=0  (b)T1=T2=To,T3=To−(3To−T),T4=0  (c)T1=To,T2=T3=To−(3To−T)/2,T4=0  (d)T1=T2=T3=To−(3To−T)/3,T4=0  (e)(When 3To≦T<4To)

Efficiency is calculated by the following seven methods (a) to (g), andthe combination that maximizes efficiency is selected.T1=To+(T−3To),T2=T3=To,T4=0  (a)T1=T2=To+(T−3To)/2,T3=To,T4=0  (b)T1=T2=T3=To+(T−3To)/3,T4=0  (c)T1=T2=T3=To,T4=To−(4To−T)  (d)T1=T2=To,T3=T4=To−(4To−T)/2  (e)T1=To,T2=T3=T4=To−(4To−T)/3  (f)T1=T2=T3=T4=To−(4To−T)/4  (g)(When 4To≦T)

Efficiency is calculated by the following four methods (a) to (d), andthe combination that maximizes efficiency is selected.T1=To+(T−4To),T2=T3=T4=To  (a)T1=T2=To+(T−4To)/2,T3=T4=To  (b)T1=T2=T3=To+(T−4To)/3,T4=To  (c)T1=T2=T3=T4=To+(T−4To)/4  (d)

Next, general expressions for torque distribution when n motors M aregiven.

(When T<k·To, (k=1))T1=T,T2=T3= . . . =T _(n)=0(When (k−1)·To≦T<k·To, efficiency is calculate by the following (2k−1)methods, and the combination that maximizes efficiency is selected (k=2to n))

T 1 = To + (T − (k − 1) ⋅ To)/1, T 2 = T 3 = … = T_(k − 1) = To, T_(k) = T_(k + 1) = … = T_(n) = 0T 1 = T 2 = To + (T − (k − 1) ⋅ To)/2, T 3 = T 4 = … = t_(k − 1) = To, T_(k) = T_(k + 1) = … = T_(n) = 0…T 1 = T 2 = … = T_(k − 2) = To + (T − (k − 1) ⋅ To)/(k − 2), T_(k − 1) = To, T_(k) = T_(k + 1) = … = T_(n) = 0T 1 = T 2 = … = T_(k − 1) = To + (T − (k − 1) ⋅ To)/(k − 1), T_(k) = T_(k + 1) = … = T_(n) = 0k−1 methods from the above.

T 1 = T 2 = … = T_(k − 1) = To, T_(k) = To − (k ⋅ To − T)/1, T_(k + 1) = … = T_(n) = 0T 1 = T 2 = … = T_(k − 2) = To, T_(k − 1) = T_(k) = To − (k ⋅ To − T)/2, T_(k + 1) = … = T_(n) = 0…T 1 = To, T 2 = … = T_(k − 1) = T_(k) = To − (k ⋅ To − T)/(k − 1), T_(k + 1) = … = T_(n) = 0T 1 = T 2 = … = T_(k − 1) = T_(k) = To − (k ⋅ To − T)/k, T_(k + 1) = … = T_(n) = 0k methods from the above.Together with the k−1 methods above, being the 2k−1 methods.(When n·To≦T, efficiency is calculated by the following n methods, andthe combination that maximizes efficiency is selected)

T 1 = To + (T − n ⋅ To)/1, T 2 = T 3 = … = T_(n − 1) = T_(n) = ToT 1 = T 2 = To + (T − n ⋅ To)/2, T 3 = T 4 = … = T_(n − 1) = T_(n) = To… T 1 = T 2 = … = T_(n − 1) = To + (T − n ⋅ To)/(n − 1), T_(n) = ToT 1 = T 2 = … = T_(n − 1) = T_(n) = To + (T − n ⋅ To)/nn methods from the above.

Thus, the torque distribution is not limited to 4-wheel drives and canbe applied to 6-wheel and 8-wheel drive vehicles, etc.

(Simplification of Torque Distribution for 4-Wheel Drives)

In general, since motor efficiency maps depicting torque-efficiencycharacteristics of the inversed U-type like that in FIG. 11A arenumerous, simplification of the torque distribution is possible.Concerning relation of efficiency with respect to torque at a givenspeed, the greater the deviation from the torque To yielding optimalefficiency is, the more efficiency drops. Therefore, the torque of eachdrive wheel is equally distributed such that the torque approaches thetorque that optimizes efficiency To. The efficiency curve in atorque-efficiency characteristics graph may be asymmetrical with respectto the torque that optimizes efficiency To as the center and therefore,there is difference between a variation of efficiency when the torque isless than the torque that optimizes efficiency To and a variation whenthe torque is greater than the torque that optimizes efficiency To.Therefore, simplified torque distribution can be performed by using theratio of the efficiency variation when the torque is less than thetorque that optimizes efficiency To to the efficiency variation when thetorque is greater than the torque that optimizes efficiency To, and therelation between the instructed torque T and the torque that optimizesefficiency To.

FIG. 16A is a diagram for explaining variation differences amongtorque-efficiency characteristics. Taking the depicted torque thatoptimizes efficiency To as the center, variation when the torque is lowis twice the variation when the torque is high. In such a case, if thedrive wheel count is 4, simplified torque distribution is performed asdescribed in (1) to (4) below. In FIG. 16A, 150 [Nm] and 75 [Nm] havethe same efficiency; 128.6 [Nm] and 85.7 [Nm] have the same efficiency;and 120 [Nm] and 90 [Nm] have the same efficiency.

(1) When T<To+2To/4 (In the example depicted in FIG. 16A, when T<150[Nm])T1=T,T2=T3=T4=0(2) When 2(To−To/4)≦T<2(To+2To/7) (In the example depicted in FIG. 16A,when 75 [Nm]·2≦T<128.6 [Nm]·2)T1=T2=T/2,T3=T4=0(3) When 3(To−To/7)≦T<3(To+2To/10) (In the example depicted in FIG. 16A,when 85.7 [Nm]·3≦T<120 [Nm]·3)T1=T2=T3=T/3,T4=0(4) When 4(To−To/10)≦(In the example depicted in FIG. 16A, when 90[Nm]·4≦T)T1=T2=T3=T4=T/4The torque-efficiency characteristics in FIG. 16A use the samecharacteristics in FIG. 11A with respect to which torque distribution inthe case of torque-efficiency characteristics of the inversed U-type wasdescribed. In substituting a value of 100 to 200 [Nm] for T in the abovecase-determined expressions, when T<150 [Nm], (T1,T2)=(T,0) is theoptimal efficiency distribution; and when 150 [Nm]≦T<257.2 [Nm],(T1,T2)=(T/2,T/2) is the optimal efficiency distribution. Therefore, thetorque distribution for torque-efficiency characteristics of theinversed U-type can be confirmed to coincide with the results depictedin FIGS. 12A to 12F.Thus, if it is known that the torque-efficiency characteristics are ofthe inversed U-type, optimal torque distribution becomes possible by asimple method of torque distribution as that described.(Simplification of Torque Distribution for n-Wheel Drive Seen in TypicalSystem)

FIG. 16B is a diagram for explaining deviation from the torque thatoptimizes efficiency in torque-efficiency characteristics. Here, k:drive wheel count; X: deviation from the torque that optimizesefficiency To, for low torque side; Y: deviation from the torque thatoptimizes efficiency To, for high torque side; a: (variation for hightorque side)/(variation for low torque side); and case-determinedexpressions for torque distribution are (1) to (3) below.

(1) When T<k·(To+(a·To)/(a·k+k+1)), (k=1)T1=T,T2= . . . =T _(n)=0(2) When k·(To−(To)/(a·(k−1)+(k−1)+1))≦T<k·(To+(a·To)/(a·k+k+1)), (k=2to n−1)T1=T2= . . . =T _(k) =T/k,T _(k+1) = . . . =T _(n)=0(3) When n·(To−(To)/(a·(n−1)+(n−1)+1))≦T, (k=n)T1=T2= . . . =T _(n−1) =T _(n) =T/n

In the case of driving by k+1 wheels, the torque value for which thesame efficiency yielded as in a case of driving by k wheels is theboundary value of the case-determined expressions (1) to (3) above.(k+1)·(To−X)=k·(To+Y)  (15)Y=a·X  (16)In solving expressions (15) and (16) to obtain X and Y, the following isobtained.X=To/(a·k+k+1)  (17)Y=(a·To)/(a·k+k+1)  (18)Case-determined expressions in the case of n-wheel drive become possibleusing expressions (17) and (18).(Dynamic Torque Distribution)

Next, an example of performing dynamic torque distribution according tovariations in speed will be described since the torque that optimizesefficiency To differs according to the speed of the vehicle (drivewheel). FIG. 17 is a diagram for explaining dynamic torque distributionaccording to travel pattern. The horizontal axis represents rotationalspeed and the vertical axis represents torque, and on a motor efficiencymap, a travel pattern of the vehicle is depicted.

In the case of the travel pattern depicted in FIG. 17, torquedistribution where T1=T, T2=T3=T4=0 is assumed until point A is reachedduring acceleration. Torque distribution where T1=T2=T/2, T3=T4=0 isassumed from point A to point B. Torque distribution where T1=T2=T3=T/3,T4=0 is assumed from point B to point C. Torque distribution whereT1=T2=T/2, T3=T4=0 is assumed from point C to point D. Beyond point D,torque distribution where T1=T, T2=T3=T4=0 is assumed. In this manner,since torque distribution that is optimal with respect to temporallychanging speed and load torque is continually performed, dynamic torquedistribution can be performed by control over a broad spectrum.

FIG. 18 is a graph depicting optimal drive wheel count data set based onthe rotational speed-instructed torque. Optimal torque distribution canbe performed in real-time during travel by creating a chart orcalculation formula for obtaining the drive wheel count that maximizesoverall efficiency, according to the speed of the vehicle (drive wheel)and the instructed torque T.

For example, dynamic torque distribution in the case of the travelpattern depicted in FIG. 18 will be described. Until point A is reached,the torque distribution is T1=T, T2=T3=T4=0, i.e., 1-wheel drive. Frompoint A to point B, the torque distribution is T1=T2=T/2, T3=T4=0, i.e.,2-wheel drive. From point B to point C, the torque distribution isT1=T2=T3=T/3, T4=0, i.e., 3-wheel drive. From point C to point D, thetorque distribution is T1=T2=T3=T4=T/4, i.e., 4-wheel drive. From pointD to point E, the torque distribution is T1=T2=T3=T/3, T4=0, i.e.,3-wheel drive. After point E, the torque distribution is T1=T2=T/2,T3=T4=0, i.e., 2-wheel drive.

The result of the algorithm of the described torque distribution, i.e.,motor efficiency map, need not be retained in memory as it suffices toretain in the memory as the graph depicted in FIG. 18 or a calculationformula, the drive wheel count and torque values of the drive wheelsoutput for the input speed and torque.

According to the first example, by drawing on the motor efficiency map,a performance curve that is sloped and takes into consideration sliprate, the operating point of the rotational speed and torque can beaccurately detected. Therefore, the calculation of efficiency by torquedistribution can be performed more accurately. Further, optimal torquedistribution can be performed with respect to each drive wheel. Duringtravel where the total torque for the left drive wheels and the totaltorque for the right drive wheels differ, the angle of the steeringwheel 302 and the angle of the vehicle are detected by the yaw ratesensor 310 and if the difference is judged to be large, the torquedistribution is adjusted such that the left/right torque differencedecreases to secure stability while the vehicle is in motion.

Second Example Configuration to Improve Overall Efficiency

In the second example, a configuration to improve overall efficiencywill be described. FIG. 19 is a diagram for explaining overallefficiency. The vehicle travels by driving motors M by power suppliedfrom the battery 312. The motor M sustains loss such as copper loss bycoil resistance or iron loss by eddy current or magnetic hysteresis.Efficiency from the supply of power until the motor M output is theefficiency of the drive system. A vehicle 1900 that travels by thedriving force of the motor M, in actuality, has a propulsion system 1901that propels the vehicle 1900 by receiving the output from the motor Mand the driving rotation of the tires. The propulsion system 1901 alsosustains loss consequent to slippage between the tires and the roadsurface. The efficiency from the output of the motor M until output aspropulsion power is the efficiency of the propulsion system. The overallefficiency of the vehicle is indicated by the efficiency of drivesystem×the efficiency of the propulsion system.

The driving efficiency is expressed as the driving efficiency ηd=motor Moutput/power supplied from the battery 312=(T×ω)/(V×I).

The torque distribution described in the first example concerns drivingefficiency. In the second example, configuration is described that byimproving the efficiency of the propulsion system, maximizes the overallefficiency.

A driving force Fd per drive wheel is expressed by the followingexpression.Fd=μ·N  (19)(μ: friction coefficient; N: normal force)Therefore, Td=Fd·r=μ·N·r  (20)(r: tire radius)

The efficiency ηΔ of the propulsion system is:ηΔ=propulsion power/motoroutput=(Fd·v)/(Td·ω)=(Fd·v)/(Fd·r·ω)=v/(r·ω)=v/v _(w)  (21)(v: vehicular speed [m/s]; v_(w): wheel speed [m/s])Further, the slip rate λ is expression by expression (11). Therefore,the efficiency ηλ of the propulsion system can be expressed using theslip rate λ.∴ηΔ=1−λ  (22)

From the characteristics of the slip rate and friction coefficient inFIG. 6, the slip rate λ can be regarded as a function of the frictioncoefficient μ and when expressed with λ=f(μ), the efficiency ηλ of thepropulsion system can be expressed as follows.ηλ=1−λ=1−f(μ)=1−f(Td/(N·r))  (23)∵μ=Td/(N·r)  (24)

From expression (20), when N is constant and Td increases, μ increases.From the relation depicted in FIG. 6, within a range where λ is 0.2 orless and μ increases, λ increases. Thus, 1−λ decreases.

FIG. 20 is a diagram depicting a torque-propulsion efficiency relationwhen the normal force is constant. In this case, as depicted, if thetorque Td increases, the propulsion efficiency ηλ decreases and thedegree of the decrease increases at the torque Td increases. In otherwords, if the torque Td of the drive wheel increases, the propulsionefficiency ηλ decreases; and if the torque Td of the drive wheeldecreases, the propulsion efficiency ηλ increases. In this manner, whenthe torque distribution described in the first example is performed, toimprove the overall efficiency, not only the driving efficiency, butalso the propulsion efficiency has to be considered. The propulsionefficiency ηλ, as described, is not limited to a configuration that usespreliminarily created chrematistics maps of the slip rate λ—the frictioncoefficient μ retained in memory. As another configuration, for example,as the vehicle travels, the vehicular speed at that time can be detectedby a sensor (or estimated by calculation) and further, the speed of thedrive wheel can be detected by a sensor and the slip rate λ can beapproximated by calculation to be used as parameters of the propulsionefficiency ηλ.

(Variation of Propulsion Efficiency Consequent to Load Variation)

By transforming expression (20), which represents torque, the followingexpression is obtained.μ=Td/(N·r)  (25)

FIG. 21 is a diagram depicting states in which the normal force differsaccording to drive wheel. From expression (25), if a given drive wheelis provided a given instructed torque, the friction coefficient μ variesaccording to the variation of the normal force N from the road surfaceto the tire. As depicted in FIG. 21, when the vehicle is climbing ahill, on a slope, accelerating, turning, etc., the load balance of thevehicle changes and if the normal force N of a given drive wheeldecreases, the friction coefficient μ increases, and the slip rate λalso increases. As a result, 1−λ in expression (22) decreases and theefficiency ηλ of the propulsion system decreases.

FIG. 22 is a graph depicting torque-propulsion efficiency according tonormal force. From expression (23), if the normal force N increases, theefficiency ηλ of the propulsion system increases, and the degree bywhich the efficiency ηλ of the propulsion system decreases accompanyingan increase in the torque Td becomes gradual. Therefore, if the normalforce N of the drive wheels varies, for a drive wheel for which thenormal force N increases, the efficiency ηλ of the propulsion systemincreases; and for a drive wheel for which the normal force N decreases,the efficiency ηλ of the propulsion system decreases. Consequently, byincreasing the torque of a drive wheel subjected to a large load anddecreasing the torque of a drive wheel subjected to a small load, thepropulsion efficiency can be improved.

(Overall Efficiency)

FIG. 23 is a diagram for explaining overall efficiency of the entirevehicle. The instructed torque T is distributed as T1, T2, T3, T4 to thedrive wheels FL, FR, RL, RR of the vehicle 300. The overall efficiencyof the vehicle 300 can be obtained by a summation of torque distributionratios of the drive wheels FL, FR, RL, RR×the driving efficiencyη_(d)×the propulsion efficiency ηλ.

Taking the drive wheel FL in FIG. 23 as an example, using a motorefficiency map 2301 and the torque T1 distributed to the drive wheel FL,the driving efficiency η_(d)1 can be obtained based on the rotationalspeed ω1. Further, based on a torque-propulsion efficiencycharacteristic curve 2302, the propulsion efficiency ηλ1 can be obtainedfrom the torque T1 distributed to the drive wheel FL. Similarly,concerning the other drive wheels FR, RL, RR, the respective drivingefficiencies η_(d)2, η_(d)3, η_(d)4, and the propulsion efficienciesηλ2, ηλ3, ηλ4 can be obtained.

The efficiency η total can be obtained by the following expression.ηtotal=(T1/T)·η_(d)1·ηλ1+(T2/T)·η_(d)2·ηλ2+(T3/T)·η_(d)3·ηλ3+(T4/T)·η_(d)4·ηλ4  (26)

By detecting or estimating the normal force of the drive wheels FL, FR,RL, RR and performing torque distribution according to the normal forcesuch that the value of expression (26) is maximized, the propulsionefficiency can be improved.

FIG. 24 is a block diagram depicting a functional configuration of thetorque distribution apparatus according to the second example.Components identical to those depicted in FIG. 1 are given the samereference numerals used in FIG. 1. The distributing unit 107 receivesinput of the normal force Nfl, Nfr, Nrl, Nrr of the drive wheels FL, FR,RL, RR from the sensors 308 a to 308 d (refer to FIG. 3), and based onthe normal force of each drive wheel, the distributing unit 107increases the torque distribution ratio of a drive wheel for which thenormal force N (load) is large, and decreases the torque distributionratio of a drive wheel for which the normal force N(load) is small.

(Calculation Example of Overall Efficiency)

FIGS. 25A and 25B are diagrams depicting calculation examples of overallefficiency. In each, the weight of the vehicle 300 is 1600 [kg]; theinstructed torque T is 800 [Nm]; the load balance moves towards the rearconsequent to the climbing of a hill or acceleration; the load of eachfront wheel is 300 [kg]; and the load of each rear wheel is 500 [kg].

In the example depicted in FIG. 25A, the torque distribution of thedrive wheels is 200 [Nm] and 4-wheel drive is assumed. Using a motorefficiency map 2501, based on the torque Td=200 [Nm] distributed to thefront wheels FL, FR and the rotational speed ω=80 [rad/s], the drivingefficiency η_(d)=0.82 is obtained. Further, based on torque-propulsionefficiency characteristics 2502, the propulsion efficiency ηλ=0.976 isobtained from the torque Td=200 [Nm] of the drive wheel FL. Similarlyconcerning the rear wheels RL, RR, the driving efficiency η_(d)=0.82 andthe propulsion efficiency ηλ=0.986 are obtained. This result, the totalefficiency ηtotal, isηtotal=((200/800)·0.82·0.976)·2+((200/800)·0.82·0.986)·2=0.80442, basedon expression (26).

In the example depicted in FIG. 25B, torque distribution of the rearwheels is 400 [Nm] and 2-wheel drive is assumed. Using the motorefficiency map 2501, the driving efficiency η_(d)=0.69 is obtained basedon the torque Td=0 [Nm] distributed to the front wheels FL, FR and therotational speed ω=80 [rad/s]. Further, based on the torque-propulsionefficiency characteristics 2502, the propulsion efficiency ηλ=1 isobtained from the torque Td=400 [Nm] distributed to the drive wheel FL.Similarly, concerning the rear wheels RL, RR, the driving efficiencyη_(d)=0.93 and the propulsion efficiency ηλ=0.971 are obtained. Thisresult, the total efficiency ηtotal, isηtotal=((0/800)·0.69·1)·2+((400/800)·0.93·0.971)·2=0.90303, based onexpression (26).

Without limitation to the calculation examples above, if the normalforce against the drive wheels differs, the driving efficiency and thepropulsion efficiency can be calculated for each drive wheel andtherefore, based on the driving efficiencies and propulsionefficiencies, the overall efficiency can be calculated. Compared toselecting a combination for which the torque distribution algorithm ofthe first example has the optimum drive system efficiency, a combinationfor which the torque distribution algorithm of the second example hasthe optimum overall efficiency is selected.

According to the second example described above, similar to the firstexample, by drawing on motor efficiency map, a sloped performance curvethat considers slip rate, the operating point of the rotational speedand the torque can be accurately detected. Thus, the calculation ofefficiency by torque distribution can be performed more accurately.Further, optimal torque distribution with respect to the drive wheelscan be performed. In addition, in the second example, since the normalforce (load) at the drive wheels is considered, the efficiency of thepropulsion system can be detected accurately, and the overall efficiencycan be improved. Further, optimal torque distribution that improvesoverall efficiency with respect to the drive wheels can be performed.Similar to the first example, when the vehicle travels with torquevalues that differ for the total torque of the left-side drive wheelsand the total torque of the right-side drive wheels, the angle of thesteering wheel 30 and the angle of the vehicle from the yaw rate sensor310 are detected and if it is judged that the difference thereof isgreat, it suffices to adjust the torque distribution amount such thatthe left-right torque difference decreases to establish propulsionstability.

The methods related to torque distribution described in the presentembodiment may be implemented by executing a prepared program on acomputer such as a personal computer and a workstation. The program isstored on a computer-readable recording medium such as a hard disk, aflexible disk, a CD-ROM, an MO, and a DVD, read out from thecomputer-readable medium, and executed by the computer. The program maybe distributed through a network such as the Internet.

EXPLANATIONS OF LETTERS OR NUMERALS

-   -   100, 400 torque distribution apparatus    -   101 instructed torque acquiring unit    -   102 a vehicular speed detecting unit    -   102 b drive wheel speed detecting unit    -   103 slip rate calculating unit    -   104 motor efficiency map    -   105 efficiency map acquiring unit    -   106 calculating unit    -   107 distributing unit    -   108 control unit    -   300 vehicle    -   301 controller    -   307 a to 307 d (rotational speed) sensor    -   308 a to 308 d (normal force) sensor    -   309 acceleration sensor    -   310 yaw rate sensor    -   312 battery    -   FL, FR, RL, RR drive wheel    -   M (M1 to M4) motor    -   INV inverter

The invention claimed is:
 1. A torque distribution apparatus distributesan input instructed torque to motors connected to drive wheels, thetorque distribution apparatus comprising: an instructed torque acquiringunit that acquires the instructed torque input; an efficiency mapacquiring unit that acquires a motor efficiency map that corresponds tothe motors; a vehicular speed detecting unit that detects vehicularspeed of a vehicle equipped with the motors; a drive wheel rotationalspeed detecting unit that detects drive wheel rotational speed of thedrive wheels; a slip rate calculating unit that based on the vehicularspeed and the drive wheel rotational speed, calculates a relationalexpression of a slip rate at the drive wheels and a frictioncoefficient; a calculating unit that based on the relational expressionof the slip rate and the friction coefficient, creates a performancecurve expression that indicates relations between torque and the drivewheel rotational speed that includes the slip rate, superimposes theperformance curve expression on the motor efficiency map, creates anefficiency variation expression that indicates for each vehicular speed,the torque and efficiency values of the motor efficiency map, andcalculates a torque that optimizes efficiency, from the efficiencyvariation expression on the performance curve; a distributing unit thatbased on the instructed torque and the torque optimizing efficiency,calculates a torque distribution value for each of the motors; and acontrol unit that within a range of the slip rate being 0 to 0.2 andbased on the calculated torque distribution values, controls torquedistribution to each of the motors.
 2. The torque distribution apparatusaccording to claim 1, wherein the motors are each an in-wheel motorconnected to the drive wheels, and the control unit controls the torquedistribution to each of the in-wheel motors.
 3. The torque distributionapparatus according to claim 1, wherein the motor efficiency map is anefficiency map that includes inverters connected to the motors.
 4. Atorque distribution method of distributing by a torque distributionapparatus, an input instructed torque to motors connected to drivewheels, the torque distribution method comprising: acquiring theinstructed torque input; acquiring a motor efficiency map thatcorresponds to the motors; detecting vehicular speed of a vehicleequipped with the motors; detecting drive wheel rotational speed of thedrive wheels; calculating based on the vehicular speed and the drivewheel rotational speed, a relational expression of a slip rate at thedrive wheels and a friction coefficient; creating based on therelational expression of the slip rate and the friction coefficient, aperformance curve expression that indicates relations between torque andthe drive wheel rotational speed that includes the slip rate,superimposing the performance curve expression on the motor efficiencymap, creating an efficiency variation expression that indicates for eachvehicular speed, the torque and efficiency values of the motorefficiency map, and calculating a torque that optimizes efficiency, fromthe efficiency variation expression on the performance curve;calculating based on the instructed torque and the torque optimizingefficiency, a torque distribution value for each of the motors; andcontrolling within a range of the slip rate being 0 to 0.2 and based onthe calculated torque distribution values, torque distribution to eachof the motors.
 5. A torque distribution value generation method ofgenerating by a torque distribution value generating apparatus, a torquedistribution value for distributing an input instructed torque amongmotors connected to drive wheels, the torque distribution valuegeneration method comprising: acquiring the instructed torque input;acquiring a motor efficiency map that corresponds to the motors;detecting vehicular speed of a vehicle equipped with the motors;detecting drive wheel rotational speed of the drive wheels; calculatingbased on the vehicular speed and the drive wheel rotational speed, arelational expression of a slip rate at the drive wheels and a frictioncoefficient; creating based on the relational expression of the sliprate and the friction coefficient, a performance curve expression thatindicates relations between torque and the drive wheel rotational speedthat includes the slip rate, superimposing the performance curveexpression o the motor efficiency map, creating an efficiency variationexpression that indicates for each vehicular speed, the torque andefficiency values of the motor efficiency map, and calculating a torquethat optimizes efficiency, from the efficiency variation expression onthe performance curve; and obtaining within a range of the slip ratebeing 0 to 0.2 and based on all the instructed torque and the torquethat optimizes efficiency, each torque distribution value correspondingto the instructed torque and wheel speed.
 6. A computer-readablerecording medium storing a program causing a computer to execute thetorque distribution method according to claim
 4. 7. A computer-readablerecording medium storing a program causing a computer to execute thetorque distribution value generation method according to claim 5.